Electron discharge devices



April 1, 1958 A. H. w. BECK 2,829,299

ELECTRON DISCHARGE DEVICES Filed Aug. 1, 1950 5 Sheets-Sheet l Inventor Ammo H. w. sEcK Attorney April 1 Filed Aug. 1, 1950 5 Sheets-Sheet 2 -l|ll|ll. III llllul ll'llllnl Inbentor ARNOLD H- W. BECK A ltorney April 1, 1958 A. H. w. BEQK ELECTRON DISCHARGE DEVICES 3 Sheets-Sheet 5 Filed Aug. 1. 1950 Qfl 1 I .llllalll-ll.

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,mo uaM q rllllll lill 33 mxEa I nvenlor ARNOLD H. W- BECK y Attorney ELECTRON DISCHARGE DEVICES Arnold Hugh William Beck, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application August 1, 1950, Syial No. 176,925 Claims priority, application Great Britain August 12, 1949 5 Claims. (Cl. 315-35) The present invention relates to electron beam arrangements where the beam current is limited by space charge and is particularly concerned with the focusing of electron beams through devices such as electron velocity modulation tubes where the maximum possible beam current is required.

In most devices utilising an electron beam, the design problem is that of forcing a beam through a tunnel without loss of electrons to the walls thereof. Heretofore, beam focusing arrangements have been either of the magneticor of the electrostatic kind. In the case of electrostatic focusing, if we assume that we'are able to set up electric fields which cause electrons emitted from a source, such as a thermionic cathode, to converge so as just to graze the entrance aperture of the tunnel, the mutual repulsion between the electrons in the field free space within the tunnel will cause their eventual divergence so that a throat is formed at which the electron beam has a minimum cross-section and the electrons are travelling substantially parallel to the axis of the tunnel. The profile of the beam is symmetrical to either side of the throat. The maximum current that can be forced through the tunnel without collection by the walls is such that the electron beam just grazes the entrance and exit apertures. I

Thus, for any given input aperture and initial axial electron velocity there is a maximum current that can This is true whatever the shape of the cross-section of the beam and also applies to systems having radial symmetry about a central cathode so as to produce a fanshaped, radiating electron beam. For a circular tunnel of diameter d and length I it is known that where I is the beam current measured in amperes and V the beam potential measured in volts. For a rectangular tunnel of cross-section a b the present inventor has shown that the maximum current is given by diameter of the input aperture is 2.4 times the diameter of the throat.

If magnetic focusing be employed, the individual electrons travel in helical paths (assuming that at the entrance to the tunnel the beam is strictly parallel to the tunnel axis) and in theory, given a 'sufliciently strong magnetic held, the beam cross-section may be made to remain constant along any length of tunnel. In practice, there United States Patent 0 be projected through the tunnel without loss to the Walls.

2,829,299 Patented Apr. 1, 1958 will be some electrons having outward radial velocity tion, with the result that a longitudinal cross-section of the beam is bounded by a pair of semi-cycloids and the magnetic field can be adjusted so that the beam has minimum transverse cross-sections at the entrance and exit apertures. The beam potential, however, or what amounts to the same thing, the axial velocity of the individual electrons, is not constant across any transverse cross-section of the beam but falls to a minimum at the center of the cross-section. If the potential falls to zero, i. e. the electrons at the center have zero axial velocity, we are left with a stationary space charge or virtual cathode. Any further attempt to force more electrons of the same initial axial velocities through the tunnel results in a denser charge formation without increase of the current issuing fromthe exit aperture of the tunnel. For most practical purposes, when the beam is to be used in an electron velocity modulation device, the potential fall on the axis must be limited to some 10%, and for this .value of potential variation in a cylindrical tunnel using magnetic focusing, the maximum current that can be projected through the tunnel is given by It is to be noted that, provided the length of the tunnel is greater than its diameter, this maximum current is independent of the tunnel length; on the other hand the magnetic field required to prevent the beam from spreading is a function of the current.

In comparing electrostatic and magnetic focusing, We see that while in the former case, for a given tunnel diameter and beam voltage, the length of the tunnel is limited by the current which is to be passed through it, a convergent beam is required at the tunnel entrance. This means that We can employ a cathode of larger area than the tunnel cross section with consequent reduction of cathode loading. With magnetic focusing, on the other hand, we are not restricted as to the length of tunnel; but the beam must be parallel to. the axis of the tunnel at its entrancewhi' ch implies that the cathode area must not be greater than that of the cross-section of the tunnel entrance. Hence, with magnetic focusing,

, for the same voltage and tunnel diameter we must have a higher cathode loading than in the electrostatic case.

The present invention combines electrostatic and mag netic focusing so as to enable a reduction in cathode loading by using a convergent beam for entry into the system, while allowing a tunnel to be used whose length is not dependent upon the current to be forced through it.

Accordingly, the present invention provides an electron beam focusing arrangement adapted to focus a spacecharge-limited current through a tunnel in which divergence of the beam is counteracted by means of an axial magnetic field, characterised in this, that the beam is electrostatically focused to converge from an electron source and to be substantially parallel to the axis of the tunnel at the entrance aperture thereof, the beam being substantially free from the influence of the said magnetic field prior to crossing the said entrance aperture. -This last condition is essential if we wish to employ purely electrostatic means to guide the beam into the tunnel. The most direct way of ensuring this electromagnetic screening is to'place the electrostatic focusing portion of the system within one of the pole pieces between which themagnetic field is set up.

The invention will be described with reference to the accompanying drawings in which:

Fig. 1 illustrates diagrammatically, for purposes of exsectional view of an electron beam focusing system according to the present invention, and

Fig. 3 shows a. longitudinal cross-section in part dingraminatic. of an embodiment of the inventiouin an tron velocity modulation device, and 5 Fig. 4 shows it longitudinal cross-section in part grummntic, of another embodiment.

in Fig. l reference numeral 11 indicates a tunnel which may form, for example, it drift tube in locity modulation device cc-tn u s rr interaction between the beam and electron-tn takes place. The beam is converged into the the tunnel by the el 'ztron gun d, which is shown at: prising a cathode 5i and locus cylinder 6. electrode is indi beam is shown at l; and nnrrt throat having .1 diameter a. filo of the beam just gru lures at the gaps 2 nu? It. norm above, since we have here d. .l (i us me or of the thront ra her than that 02 the tunnel, the expression for the n wimum current through the tun; become.

In Fig. 2 the tunnel 9 of ion between two pole pieces in ally N and S resucctively, l'c for interaction ,1: "l. Pol and contains an elccumn gun cornpriv and focusing electrode 16. The inn-r piece 10 may conveniently be made to form the of the electron gun, and =0 to term tunnel of entrance diameter ct" so that the elect on whose profile is indicated 37 may term an el throat of diameter d at the gun 1?. S the space inwn hollow cathode. :S 30

wall may conveniently be eter d to (1 without in on way the beam. "he clec'tron ing to well mo converging Cull. tunnel. The CO) altering the one: d t

be designed for the some purg w front \Vtlll of the pole piece piece forms one half til shown in Fig. l. role contain nrecess 18 in which the elect lected. 'l' to dinmcte l -l t that of the entrance for the divergence or Elect velocities at the cntrnncc inc connection with magnetic focusiri locity modulation device the l. the beam at the requirements of the tube being determined by quired (i. e. the factor usua system in which L is 5 cm. on; 1) would be about 2.5 mm

As an example of a 1 ing to the present inventi electrostatic or magnetic to some that we wish to limit the Lmilll'l voltage to it From Equation 1, in the r case we ililti that the maximum current that e ough ct tunnel of 5 centimeters length is 9. magnet c case, allowing for a 0% fall in potential in the be we find, from Equation 3, that the maximum curt 197 mu. Thus the elcctr 1r much higher current i t a current would be filOLlbiill c, since, containing n nc: diameter of 2.5 mm. (equal to the main tunnel) it would rous are tin imam it i2. One half of the nch, with 1330 volts the pole piece width [/2 is not 'ns to def. c electrons.

"n principles with on gun.

l-in present crned. which will otojc'; Huu Zllll volts into the tunnel lly the decrease in loading it ic'ce, connnonly sity at the clectr us that at the cutho riple the current dc..- 0111. so that the cathod 0.32 tit/cm? which is well ol moder tic-co as a rensouztole the. current densities of working part of the an cxun e ecausc it is re treatment. Gther effectively and there is or the maximum ratio of cathode men to il'li'i, can be obtained.

In the embodiment of Fig. 3, the invention is shown applied to it two resonator ltlystrou. The tunnel 9 of Fig. 2 is replaced by a drift tube 19, separating two resonant cavities 2t? and These cavities are formed between pairs of metal discs 22 and 23, the disc 22 forming the outer wall being made of nickel-iron alloy having substantially the some cocilicient of expansion as glass, such as; that sold under the registered trademark Ciuseal, while the disc 23 may be of copper. The discs are clamped to annular metal collars and 25 which may be provided with tuning screws 25 for adjusting the resonant frequency of the cavities and Glass sleeves 27, 28 and 29 are sealed between the tespective discs as shown to form an envelope for the device. Tubular extension members 36 and 31 secured to the respective plates 22 are aligned with the drift tube 19 and form continuations thereof leaving gaps 32 and 33 in which interaction between the electron beam and the electromagnetic field in the respective cavities, 2d ind 21 may occur. Hollow :t is about 8 a./ about my Qt t.,-

loading would only be 1; handling pole pieces 3? and 35, corresponding to the pole pieces.

.l and of F 2, are secured to the respective plates 22 at either end of the device. The pole piece 3:! houses an indirectly heated cathode 35 and a focusing cylinder 37, which components are shown diagrammatically on the drawing. The hermetic enclosure of the pole piece 34 is completed by an alloy sltirt 33 sealed to the material of the pole piece and to t: lass base 39 carrying an exhaust lllblllttliOil "ii? and ledtli; ll or the electron gun electrodes. Pole piece 35 is closed by an end plate 42 which forms the collector electrode for the electrons proted through the device. 'l" magnetic circuit is commeans of u permanent magnet or solenoid indiouted at 43 and ed by means of yoke arms 44 to ective pole es. The dimensioning and operation sl-Lillcd in the art of the device will be evident: to those from foregoing discussion with reference to Fig. 2.

that me pole piece 34 lo brovide adequate mag l'or the electron gun.

t to note hoxv lt is imports In the embodying of Fig. 4, the invention is shown as 9 applied to a travelling wave tube. The tube comprises a glass sleeve 46 which is sealed by short Cinseal cylinders 47 to the pole pieces 48 and 49. A helix 50, terminating in pick-up posts or probes 51 and 52 secured to the pole pieces, is positioned in the sleeve 46 .by means of quartz support rods 53 which are located in the pole piece and faces.- The travelling wave tube projects at either end through rectangular wave guides indicated at 54 and 55.

The pole piece 48 houses an electrostatic gun comprising an indirectly heated cathode and focusing cylinder shown diagrammatically at 56 and 57 respectively, these members being located by mica Washers 58. As in the previous example, the pole piece 48 isv closed by means of an alloy spinning 59 sealed to a glass base 60 carrying an exhaust tubulation 61 and electrode leads 62. At the other end of the tube the pole piece 49 comprises a cylindrical member 63 in which iron or Cinseal blocks 64 and 65 may be brazed, these blocks being hollowed out to provide a chamber 66 the walls of which constitute the electron collector electrode.

As in Fig. 2 the magnetic circuit is completed byyoke arms 67 and a permanent magnet or solenoid 68.

While the principles of the invention have been described above in connection with specific embodiments,

and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention. In particular the same principles may be used to obtain rectangular cross-section beam of high current density, if cylindrical lenses are substituted for the spherical lenses discussed above. I

What I claim is:

1. An electron discharge device having an electron beam source, a tunnel member for the flow of an electron beam therethrough, and means to provide a magnetic field coaxially of said tunnel member to minimize divergence of said beam within said tunnel member, said beam source comprising an electron emitter of materially greater cross-sectional area than that of said tunnel memher, electrostatic focusing means to converge the electrons from said emitter into a beam of a cross-sectional area to enter into said tunnel member for substantially parallel flow at the entrance thereof, and a shield of magnetic material for shielding said beam from influence by said magnetic field prior to entrance into said tunnel member.

2. An electron discharge device according to claim 1, wherein the means for producing said magnetic field includes a' pair of pole pieces at opposite ends of said tunnel and said electrostatic focusing means is contained within one of the pole pieces.

3. An electron discharge device according to claim 2, wherein said one pole piece is hollow and has an aperture forming'a second tunnel member adjacent the en- 7 trance and coaxially of the aforementioned tunnel meniber, said second tunnel member constituting said electrostatic focusing electrode for said beam and'the end of said one pole piece adjacent said second tunnel member formingsaid magnetic shielding means.

4. An electron discharge device according to claim 3, further including a pair of cavity resonators, one adjacent each pole piece and in communication'with the ends of said first tunnel member.

5. An electron discharge device according to claim 3, further including a pair of wave guides one adjacent each pole piece and in communication with the ends of said first tunnel member, and wherein the means defining said first tunnel member includes ahelical wave conductor.

References Cited in the the of this patent 1 UNITED STATES PATENTS 

